A dynamometer that tests both concentric contractions and eccentric contractions of the muscles of a user must absorb torque in the direction in which the user's limb is exerting a force against a handle or input arm of the dynamometer (so-called concentric muscular contractions) and must also absorb torque in the direction opposite to the direction the user's limb is exerting a force against the handle or input arm (so-called eccentric muscular contractions). Eccentric contraction is also known as passive ranging because the handle or input arm of the dynamometer is driven by an external source (such as a motor) to move the user's limb through the range of motion in both directions without any assistance by the user.
Some dynamometers have tested eccentric contractions by directly attaching the limb of the user to a drive source such as an hydraulic actuator or an electric motor via a gear box. Such systems are not particularly safe because the user cannot disengage his limb from connection to the drive source in case of pain or some sort of emergency situation. Use of a clutch which allows for quick limb disengagement from the drive source adds somewhat to the safety of such dynamometers, though in the view of some, there remains an element of danger to the user in passive ranging systems, regardless of the safety features.
Examples of prior art dynamometers include isokinetic dynamometers, such as that shown in U.S. Pat. No. 3,465,592 issued September 1969 to J. Perrine. During concentric contractions, isokinetic dynamometers offer controlled speed of movement and an accomodating resistance which automatically adjusts to always equal the force applied by the user. In other words, once the user accelerates his or her limb to the desired testing speed, the user will be unable to increase the speed of rotation of the input arm.
Known dynamometers which test both concentric and eccentric contractions make it very difficult for the user to accelerate his limb quickly or smoothly to the desired testing speed during concentric contractions because the dynamometer power source, whether it be hydraulic or electro-mechanical, must match the user's limb acceleration. If the user accelerates his limb at the fastest rate possible, the dynamometer power source may not accelerate as quickly, resulting in the power source resisting the user's limb acceleration. Since the range of motion of the input arm is limited, this condition results in having the user's limb moving at the desired testing speed for a lesser period of time during concentric contractions.
Ideally, a concentric-eccentric dynamometer should allow for free-limb acceleration during concentric contractions where the user can accelerate his or her limb up to the set speed of the dynamometer without any resistance from the dynamometer. Since such dynamometers are designed to test the user's musculature at set speeds, the failure to provide free-limb acceleration results in loss of test data and greater fatigue to the user. In addition, free-limb acceleration more closely simulates actual function, such as the swinging of the arms during a walking motion. Failure to allow for free-limb acceleration therefore results in a less natural limb movement for the user.
Such an ideal dynamometer should also provide as many safety features as possible during eccentric contractions.
Isokinetic dynamometers manufactured by Cybex Division of Lumex, Inc., Ronkonkoma, N.Y., such as the dynamometer which is part of the Cybex.RTM. 340 isokinetic testing system, use overrunning clutches which permit a servo motor power source to run continuously in one direction at a desired testing speed. The user is free to move the handle or input arm in either direction with no resistance until the set testing speed is reached. Once the set speed is reached, the user performs concentric muscular contractions, which are then precisely measured. However, the present Cybex.RTM. isokinetic system dynamometers only allow for concentric contractions, and do not permit testing of eccentric contractions. These current Cybex.RTM. dynamometers jam when the motor direction is reversed to provide for eccentric contractions. When the servo motor is running in the direction designed for concentric contractions, the clutches are overrunning in opposite directions to one another on two parallel shafts. The two parallel shafts are geared to an input shaft connected to the handle or input arm of the dynamometer. If the motor direction is reversed in an effort to eccentrically load a muscle, both clutches engage, each attempting to drive the input shaft of the dynamometer in an opposite direction, resulting in zero net movement, thus jamming the dynamometer.
Although a dynamometer which tests eccentric contractions is not inherently 100% safe to the user regardless of the number of added-on safety features, there remains a need in certain situations for a dynamometer which tests both concentric and eccentric contractions in as safe a manner as possible, particularly for eccentric contractions, and also allows for free-limb acceleration u to the test speed during concentric contractions.